The Hourglass

[Khandro]

I've been thinking about hourglasses (don't ask!) according to an entry in wikipedia
https://en.wikipedia.org/wiki/Hourglass the first known ones appeared in Alexandria around 150 BC but it doesn't say, -without any mechanical time keeping devices- how they would be able to check the first one was in fact of an hour's duration.
Lying awake abed, all I could think of would be to construct the glass form with its narrow waist and open ended and starting at any given point in sunlight by using a marker's shadow, pour sand through it for exactly 24 hours, and then use one twenty fourth of that sand.
Or could there have been a better way?

[albaqwerty]

cor, interesting question.
Who thought about making them with a narrow waist?

You've now got me wondering about water-clocks and sun-dials !

[aelmpvw]

didn't they have shadow clocks or sun dials to tell the time?
th making the hourglass would then be a matter trial and error

[Old_Geezer]

And there was I thinking this would be about some young lady you saw on TV.

[Old_Geezer]

Possibly trial and error. You make one and it is way out, then next time it's fairly close. After a few weeks of making them you are pretty near spot on.

It depends on how much you want it to be near perfect first time. Chances are you only wanted to measure a set period of time initially.

[jim360]

Perhaps the more contemporary name of "sand glass" gives another answer: it might not necessarily have been an hour that sand-glasses would keep the time for?

But I suspect your idea is probably reasonable. Time-keeping progresses from using the passage of the Sun (and Moon and stars) via sundials, then to more mechanical devices like water-clocks that were calibrated based on those astronomical movements, then onwards to even more mechanical devices in the Middle Ages. Each one can only be useful if it agrees with, and then improves on, the device before.

[jim360]

As a separate point, the 150 BC date for sand-glasses is disputed (although Wikipedia acknowledges this in the very same paragraph you quote from). On the other hand, water clocks existed since probably 1500BC or so, and sand-based hourglasses were definitely in use in the 1300s AD, and other sources express understandable doubt that nothing much of note in time-keeping happened between the two dates.

[Togo]

The sands of time are definitely running out. As some have said a bit of trial and error would be needed. I would also think that different sized grains of sand would run through the aperture at different rates, and that the sand would have to be dried thoroughly before being sealed into a glass.

[Khandro]

There's a few mentions of trial and error - but against what criterion?
I did first considered taking an hour from a sundial but making the judgement when to start and finish in a short period isn't going to be very accurate, maybe the first ones weren't, Wiki doesn't say how good they were if tested against a modern timepiece.
It does though say that sand wasn't the best material because of the variations in grain size and ground eggshell was sometimes used.

[jim360]

Time-keeping in the ancient world is often a hodge-podge anyway. With only the Sun, Moon and Stars to go on initially, it was always bound to be. The problem is that the length of a day turns out to vary, the length of a month turns out to vary, and the length of a year isn't set in stone either -- and anyway, none of these really agrees with each other very well. There aren't 12 lunar months in a Solar year, and there are about four or five different "years" you can define, and there aren't a whole number of days in a month or hours in a day... it's just an unholy mess.

Anyway, what you can do (and what was probably done) is say that, "when the Sun rises, and then rises again, the time between those two instants is called 'a day'. We will divide that time in to twenty-four equal parts. We want to measure one of those equal parts." This (or using Noon = highest point the Sun reaches in the course of a day, or sunset) is probably what was done to start with.

[Fitzer]

As an aside, because noon is the only point in the day one could be certain about (sun at its highest) the Royal Navy had the convention that a day started at noon and this was only abandoned in 1805

[Khandro]

Fitzer; How did the RN ascertain when the sun was exactly at it's highest point?

[aelmpvw]

that's not what set trafalgar off was it?

[jim360]

Sextants?

[Fitzer]

Sextant (and before that a cross staff), essentially when you think it is approaching noon you measure the height of the sun above the horizon until the angle starts to reduce and declare the time at the highest point to be noon. In the words of the Captain 'make it noon', this allows you to know your latitude as there were tables showing at what latitude the sun would be at x angle on y day. Once reliable chronometers had become available to enable you to know what time it was at your home base knowing the difference between local noon and home noon allowed you to work out your longitude and the world (navigationally speaking) was your oyster :-)

[Khandro]

Fitzer, Very good! and it all sounds more fun than having a quick glance at your mobile phone.
I've been thinking a lot about time; physical, emotional, comparative and spiritual, since recently reading Thomas Mann's great book, The Magic Mountain if anyone has read it they will know what I mean.